Administration of 10 to the 7th power CFU of M. pneumoniae by the intratracheal route to 3 squirrel monkeys produced a silent infection. Three subsequent challenges of these monkeys with M. pneumoniae were followed by a shorter, more restricted infection without accompanying disease. There was no evidence of transmission of the organism to broth-inoculated cagemate controls. BIBLIOGRAPHIC REFERENCES: Greenberg, H. B., Helms, C. M., Grizzard, M. B., James, W. D., Horswood, R. L., and Chanock, R. M.: Immunoprophylaxis of experimental Mycoplasma pneumoniae disease: Effect of route of administration on the immunogenicity and protective effect of inactivated M. pneumoniae vaccine. Infect. Immun. 16: 88-92, 1977. Brunner, H., Prescott, B., Greenberg, H., James, W. D., Horswood, R. L., and Chanock, R. M.: Unexpectedly high frequency of antibody to Mycoplasma pneumoniae in human sera as measured by sensitive techniques. J. Infect. Dis. 135: 524-529, 1977.